Babylonians
were very good sky observers but their ability to build models of the universe
was limited.The Greeks were always
asking why things happened the way they did, a question that would never have
occurred to Babylonians.

An
early model predicted lunar phases and eclipses.This model had to change the basic assumptions about the
structure of the universe:

The universe is
two-dimensional.Everything in the
sky is on the Celestial Sphere.

The sky is close.

Objects in the sky glow of their own
light.

Earth is the center of the Universe.

Objects in the sky move on perfect
circles.

Eclipses
are just shadows cast from one body onto another.At New Moon the moon is between the Earth and the Sun and it is
possible for the shadow of the Moon to be cast onto the Earth, producing a
solar eclipse.At Full Moon the Earth
is between the Sun and the Moon and the shadow of the Earth can fall onto the
Moon, producing a lunar eclipse.This
simple model relies on two changes in the basic assumptions:

The universe is three-dimensional -
at least the Sun has to be farther away than the Moon

The Moon is a dark body - we see
reflected light from the Sun.

Phases
of the Moon are seen to be simply geometry - as the Moon goes around the Earth,
how much of the sunlit portion do we see from the Earth.

Once
phases and eclipse are understood, the Greeks can take the next logical
step.If the shadow of the Earth
produces the lunar eclipse, we can judge the shape of the Earth.The Babylonians recorded many lunar
eclipses, but all were sections of a circle.The only shape that can only cast a circular shadow is the sphere.

If
we now know the shape of the Earth, can we measure the size of the Earth.Erastosthenes, the curator of the Greek
library in Alexandria, came across a note from the far southern city of Syene
that said, "At noon on the summer solstice, we can see the bottom of a
deep well."The Sun has to be at
the zenith at this time.But where is
the Sun in the sky at Alexandria at the same moment.Erastosthenes measured that the Sun was 7.5 degrees South of the
zenith.With these data we can estimate
the size of the Earth.The angular
difference of the Sun in the two cities is about 1/50th the
circumference of a circle.Thus the
distance between the two cities must be 1/50th the circumference of
the Earth.

Using
other measurements of the Moon at critical times in its orbit, the Greeks were
able to obtain the relative distance and sizes of the Earth, Moon, and
Sun.They saw that the Sun was enormous
compared to the Earth and the Moon, but that the Earth was larger than the
Moon.Useful in this analysis was the
fact that the angular sizes of the Moon and the Sun are almost exactly the
same.

These
advances lead to the first comprehensive models of the universe.A major player in one of the models was
Aristotle.He presented evidence that
the Earth was the stationary, center of the Universe.His model is called "Geocentric."

The Earth is massive - easier to
assume the things in the sky are moving.

It looks like the sky is moving and
we are standing still.

If the Earth were spinning, objects
would fly off of the surface.

No observed stellar parallax (the
shifting of a star position that must occur if the Earth orbits the Sun)

Aristotle
divided the universe into two realms.In the Terrestrial Realm motion could be predicted only when the
composition of the object was known.Elements had natural tendencies of motion.Earth and water tended to sink, while air and fire tended to rise.Objects that followed their natural tendencies
(natural motion) required no forces.Opposing the natural tendency (violent motion) required force.There was an overall tendency in the
Terrestrial Realm to seek rest.Likewise in the Terrestrial Realm everything was corruptible (changing).

Rules
are different in the Celestial Realm.Here objects glowed but not by fire.It was called Aether and had the property of glowing without
consuming.Motion of the celestial
objects was constant and circular.Also celestial objects were incorruptible.Objects like meteors or comets must belong
to the Terrestrial Realm in the upper Earth's atmosphere.The Geocentric model of Aristotle could not
account for retrograde motion.

Aristarchus
of Samos preferred the "Heliocentric" model with the Sun at the
center and the planets orbiting it.The
lack of stellar parallax must now be because the stars are too far away to have
percepable shifts.Thus the
Heliocentric model must be huge, an idea that ancient astronomers were not
prepared to accept.The model that
comes down through history is the Geocentric Model.

About
140 AD Claudius Ptolemy introduced a major revision of the geocentric model in
order to account for retrograde motion, the apparent occasional backwards
motion of the planets through the sky.He started by noting that the Sun and Moon showed a variation in their
apparent size as they moved around us.To account for this Ptolemy offset the Earth from the center of the
orbit of the Sun and Moon.Thus at some
places in the orbit the Sun or Moon would be closer to us and appear bigger in
the sky.

The
planets all showed retrograde motion, however.Ptolemy devised a system of circles on circles (epicycles on deferents)
to account for this motion.So tightly
held was the belief in circular motion that this system persisted into the 17th
century.

After
the fall of Rome the knowledge of the ancient world moves into the new world of
Islam.These astronomers were not model
builders, but they were very good sky observers, who expanded the Greek models.In Europe illiteracy was almost universal, communications
were poor, and the Alexandrian library was destroyed.

The
Geocentric model is later tied to the teachings of the Christian religion
through the work of Thomas Aquinas.